1. Field of the Invention
The present invention relates to an ink-jet printing apparatus and an ink-jet printing method, and in particular, to a configuration thereof to reduce a granular feeling in a printed image by an ink ejection control.
2. Description of the Related Art
In accordance with the popularization of information processing apparatus such as personal computers in recent years, printing apparatuses as image forming terminals have been rapidly developed and popularized. Among various printing apparatuses, ink-jet printing apparatuses that perform printing by ejecting inks onto a printing medium such as a paper have various advantages, and have become a mainstream in personal use. The ink-jet printing apparatuses are required to be further improved in an image quality with such extensive popularization, and for example, a quality level of an image corresponding to silver halide photography is required for the ink-jet printing apparatuses as a print system capable of printing photography easily at home. In ink-jet printing apparatuses, there has been conventionally a problem of a granular feeling in a printed image as compared with silver halide photography.
Various countermeasures for the granular feeling have been proposed. For example, ink-jet printing apparatuses having an ink system in which light cyan and light magenta inks with lower concentrations of color materials are added in addition to normal cyan, magenta, yellow and black inks, have been known. Specifically, by using such an ink system, it is possible to reduce the granular feeling by using light cyan and light magenta inks on an area of lower print density. Then, by performing printing on an area of higher print density by use of the normal cyan and magenta inks, it is also possible to realize a broader color reproducibility and smoother gradient.
Further, there is also a method for reducing the granular feeling by designing a printing apparatus so as to make dots formed with inks on a printing medium smaller in size. This is a method for reducing the volume of an ink droplet to be ejected from ejection openings of a printing head. In this case, not only are the volume of ink droplet reduced, but also a greater number of ejection openings are arrayed at a higher degree of density, which makes it possible to simultaneously obtain a high resolution image without degrading its printing speed.
As factors to emphasize the granular feeling, there are not only the above described high concentration of color material in ink and sizes of ink dots to be formed, but also bleeding and beading of ink. Since there is a limit to an amount of ink absorbed by a printing medium, for example, when inks in an amount greater than the limit are ejected onto a given area of the printing medium, ink bleeding occurs, which may appear as a granular feeling in some cases. In particular, in a case in which two or more color inks are ejected so as to be overlapped on one another in one pixel, ink bleeding due to an amount of inks being greater than a limit to an ink absorbing amount easily occurs. For example, in an apparatus using four color inks of cyan, magenta, yellow and black, in a case in which secondary colors which are red, blue and green are expressed, an ink amount to be ejected per one pixel is made greater than that in a case in which primary colors which are yellow, magenta and cyan are expressed. Further, in such a case, beading serving as a phenomenon in which a plurality of ink droplets are not absorbed exist as liquid on a paper surface, and those droplets attract each other to become greater droplets, to finally form dots greater than the original dots may be caused.
In order to suppress a worsening in a granular feeling due to these bleeding and beading, the following method has been proposed. By performing so-called multipass printing, it is possible to reduce an ink amount to be applied to a unit area on a printing medium per one scanning or unit time. Thereby, it is possible to reduce ink bleeding on a printing medium, or beading due to ink droplets being not able to be absorbed to contact with each other in a liquid state on the printing medium.
Further, as another method for reducing an ink amount to be applied to a unit area per unit time, there is disclosed in Japanese Patent Laid-Open No. H10-44473 (1998) that uses not only yellow, magenta, cyan, and black inks, but also inks of secondary colors of red, blue, and green. Specifically, at the time of printing the secondary colors of red, blue, and green, it is possible to reduce an ink amount to be applied to one pixel by respectively using the inks of red, blue, and green.
Moreover, there is disclosed in Japanese Patent Laid-Open No. 2000-229424 that a limit where the maximum ejection amounts of the respective color inks at the time of printing secondary colors are less than or equal to 100% is provided, and printing is performed by use of two colors of the primary color inks and secondary color inks.
However, in a multi-pass printing system, when the number of passes is increased in order to reduce an ink amount to be applied to a unit area on a printing medium per unit time, this may be a cause for bringing about an increase in a time required for printing, which prevents throughput enhancement and speeding up of printing. In contrast thereto, the methods disclosed in Japanese Patent Laid-Open No. H10-44473 (1998) and Japanese Patent Laid-Open No. 2000-229424 are used in place of the mode in which the number of passes is increased, to be able to reduce an ink amount to be applied per unit area. However, in a case that the number of passes for the multi-pass printing is small even if the methods disclosed in Japanese Patent Laid-Open No. H10-44473 (1998) and Japanese Patent Laid-Open No. 2000-229424 are used, the greater applying amount per one pass due to smaller number of passes may far more affect degradation in image quality than reducing applying amount of ink per a unit area by use of the secondary color of inks. Then, in such a situation, even if all the inks finally provided thereto are able to be absorbed, it may be insufficient for a printing medium to absorb inks applied in the middle of completing an image.
On the other hand, in accordance with the study of the inventors of the present application, it has been confirmed that an ink applied to a region on which an ink has already landed takes time to complete its permeation longer than that of the ink first applied to a printing medium, which easily causes beading.
FIGS. 1A and 1B and FIGS. 2A to 2F are diagrams for explaining this phenomenon.
FIGS. 1A and 1B show a state in which ink first is applied to places on a printing medium on which ink has not applied. Specifically, FIGS. 1A and 1B show a state in which a cyan pigment ink is applied to the printing medium that is a so-called glossy paper. When the cyan pigment ink 101 is applied to a printing medium 102 (FIG. 1A), a water component and a solvent of the ink permeate the printing medium and evaporate at the same time, and a solid content 103 of the pigment ink fixates to the printing medium 102 (FIG. 1B).
On the other hand, FIGS. 2A to 2F show a state in which ink is applied to a printing medium that is a glossy paper and the ink fixates thereto and other ink further is applied to the fixation place and fixates thereto. When a cyan ink 201 in a sufficient amount to cover the surface thereof is applied to a printing medium 202 (FIG. 2A), a cyan ink layer 203 is formed thereon (FIG. 2B). Then, next, when a magenta ink 204 is applied to the printing medium (FIG. 2C), a time to complete permeation of ink droplets 205 applied to the cyan ink layer 203 is made longer than that of ink droplets 206 applied to a place on which there is no ink layer (FIG. 2D). As a result, when a yellow ink 207 is applied to a place respectively adjacent to the ink droplets 205 and 206 (FIG. 2D), since the ink droplets 205 have not sufficiently permeated yet, the ink droplets 205 and the ink 207 bring about beading 208 (FIG. 2E). Then, an ink layer 210 due to the beading is formed thereon (FIG. 2F). On the other hand, since the liquid component of the ink droplets 206 has sufficiently permeated, the ink droplets 206 and the ink 207 applied later are not coupled together (FIG. 2E), and ink layers 211 in a separated state is finally formed (FIG. 2F). The ink layer 210 due to beading formed as described above appears as a granular feeling in a printed image, which brings about degradation in its image quality level. The reason for that there is a difference in the times to complete their permeation, may be that ink layers have pore structures which are densely filled more than that of a printing medium. Note that, in the description with reference to FIGS. 2A to 2F, the example in which the ink is applied to a position adjacent to the inks applied in sequence (the relationship between the ink droplets 205 and the ink 207 in FIG. 2D) has been described. However, as a matter of course, the problem of permeation speeds is not necessarily caused in only this example. In a case in which an ink ejected in certain timing is applied to the same position of an ink ejected in the previous timing to be overlapped thereon, the same problem of permeation speeds is caused. Further, in contrast thereto, even in a case that pixels to which inks are applied are not adjacent to one another, the same problem is caused in a case in which inks are applied in sequence at a distance at which the inks may contact with each other in accordance with sizes of applied ink droplets.
Moreover, in accordance with further study by the inventors, it has been made clear that, in a case that an ink is applied to a place on which an ink has already landed, times to complete their permeation differ in accordance with ink properties of inks applied in sequence. Specifically, when two or more types of inks are used, an order of overlapping the inks differs in accordance with an order of ejecting these inks, which generates a difference in times to complete overall permeation according to those orders. When the time to complete permeation is long, beading may occur.
Various reasons for that a permeation speed with respect to an ink layer differs in accordance with a type of ink, are possible. However, a filling rate for a pore structure formed by an ink solid content on a printing medium can be cited as a major factor. For example, in a case in which a pigment and resin are included in an ink, its pore structure is formed such that the resin fills among pigment particles on a printing medium. Then, a permeation speed with respect to an ink layer differs in accordance with a remaining ratio of pores which are not filled (hereinafter called a “P/B value” as well) in these pore structures. That is, the greater the P/B value is, the more the filled structure of an ink layer is sparse, and therefore, permeation of an ink to subsequently be applied is made faster.
Further, it has been known that carbon black generally used as a black pigment forms a grape cluster like structure in which primary particles fuse to attach to each other. In a case that carbon black is included in a color material of ink, a permeation speed with respect to an ink layer differs in accordance with differences of that structure as well. Here, a level of a size of a structure is denoted by DBP oil absorption amount, and the DBP oil absorption amount means a value measured in accordance with the JIS K6221 A. Specifically, an ink with high DBP oil absorption amount forms a high ink layer at the time of forming an ink layer, and a permeation speed of an ink to be applied next is made faster. On the other hand, an ink with low DBP oil absorption amount forms a highly-dense ink layer at the time of forming an ink layer, and a permeation speed of an ink to subsequently be applied is made slower.